MR22A-01 INVITED
Intermediate-spin ferrous iron in lower mantle perovskite
The lower mantle is dominated by (Mg,Fe)(Si,Al)O3 perovskite, where the ability of iron to adopt multiple valence and spin states can affect a broad spectrum of physical and chemical properties. A high- to low-spin transition of Fe2+ in (Mg,Fe)O is now well established by both experimental and computational data to occur near 50 GPa at room temperature for lower mantle compositions. However in the Earth's most abundant phase the picture is not so clear. Previous X-ray emission (XES) and nuclear forward scattering (NFS) data present conflicting results on the location, number and sharpness of the transition(s), and whether Fe2+ or Fe3+ or both are involved. To reconcile these observations, we undertook the first high-pressure high- temperature study of iron-containing silicate perovskite using combined Mössbauer and NFS techniques to determine the spin state of iron in the dominant lower mantle phase. We collected 119 57Fe Mössbauer and 32 NFS spectra of Mg0.88Fe0.12SiO3 and Mg0.86Fe0.14Si0.98Al0.02O3 perovskite using a resistively-heated diamond anvil cell at pressures up to 110 GPa and temperatures up to ca. 1000 K, combined with high-resolution X-ray diffraction of several of the same sample loadings. Spectra show the appearance of a new quadrupole doublet above ca. 30 GPa with extremely high quadrupole splitting and narrow linewidth, which can be explained only as intermediate- spin Fe2+. The pressure dependence of the average spin number calculated from the Mössbauer data is in excellent agreement with all previous XES data for both Al-free and Al-containing silicate perovskite. Our high- temperature data show that elevated temperatures stabilise the intermediate-spin state; hence Fe2+ in silicate perovskite is inferred to be predominantly in the intermediate-spin state throughout most of the lower mantle.
MR22A-02 INVITED
Elasticity of Ferropericlase at Earth's Lower Mantle Conditions
The thermoelastic properties of ferropericlase Mg(1-x)FexO (x = 0.1875) across the iron high-to-low spin crossover have been investigated by combining first principles calculations with a novel thermodynamics model of this system. At room temperature the transition is somewhat sharp and the effect on the bulk modulus is quite dramatic. Along a typical geotherm the transition should occur across most of the lower mantle with a noticeable bulk modulus reduction in the mid lower mantle. This transition should also alter noticeably the magnitude of velocity heterogeneities caused by lateral temperature changes. Research supported by NSF/EAR, NSF/ITR (VLab), and MSI(U of MN)
MR22A-03
Spin Transition Zone (STZ) in the Lower Mantle
Properties of the lower-mantle minerals such as ferropericlase [(Mg,Fe)O], silicate perovskite [(Mg,Fe)SiO3], and post-perovskite are affected by the electronic valence and spin states of iron, but the electronic states of iron under representative pressures and temperatures of the lower mantle have not yet been probed. Here the spin states of iron in lower-mantle ferropericlase and its crystal structure have been measured up to 95 GPa and 2000 K with x-ray emission and x-ray diffraction in a laser-heated diamond cell. Results show that an isosymmetric spin crossover of iron occurs over a wide pressure-temperature range extending from the middle part to the lower part of the lower mantle. The spin transition zone of iron in the lower-mantle phases significantly affects its implications for the geophysics, geochemistry, and geodynamics of the lower mantle. In particular, as the spin crossover of iron occurs in the lower-mantle minerals such as ferropericlase at high pressures and temperatures, their thermal compression curves, sound velocities, and transport properties will be continuously influenced by the ratio of the high-spin and low-spin states along the lower-mantle geotherm. Using recent results on the effects of the spin transitions on the density, elasticity, sound velocities, electrical conductivity, and strength of the lower-mantle phases, we will address how the electronic spin states in lower-mantle phases and their associated effects affect our understanding of the composition, geophysics, and dynamics of the lower mantle. This work was performed under the auspices of the U.S. DOE by UC\LLNL under Contract W-7405-Eng- 48.
MR22A-04
Fe Spin Crossover in Lower Mantle Materials: A First-Principles Study
Ab initio calculations of the zero-temperature composition dependent Fe high- to low-spin transition in (Mg1-x,Fex)SiO3 perovskite and (Mg1-x,Fex)O ferropericlase are presented and comparisons are made between the two structures. Opposite trends in spin crossover dependence on composition are obtained. The possible role of high, intermediate, and low spin states is also discussed. We explain the large spread in computational Fe spin transition pressure in perovskite seen in the literature by differences in calculation parameters such as exchange correlation type, exchange correlation parametrization, pseudopotential choice, and correlation corrections (LDA+U). We also find the spin transition pressure to be very sensitive to materials systems parameters such as structural relaxation (especially for high Fe content), composition, local chemical ordering, valence, and magnetic ordering. Our results indicate that there needs to be consistency in both calculation and materials systems parameters when comparing calculation results and when comparing calculations to experiments.
MR22A-05
The Electronic Structure of Iron in (Mg,Fe)SiO3 Perovskite and Post-Perovskite Under Megabar Pressures
The bottom few hundred kilometers of Earth's mantle, termed the D" layer, represents one of the most extreme compositional and thermal boundary layers within our planet, where the solid silicate and (Mg,Fe)O dominant mantle is in contact with with the iron-dominant outer core. Knowledge of iron's electronic structure in this material, which in turn may influence elastic and transport properties, provides important constraints on our understanding of this boundary layer. (Mg,Fe)SiO3 has been suggested to crystallize in the post-perovskite structure under the pressures and temperatures of Earth's D" layer, and is therefore the focus of many current experimental and theoretical investigations. We have determined the electronic structure of iron in (Mg,57Fe)SiO3 perovskite and post-perovskite at megabar pressures through direct measurements of iron's hyperfine fields using synchrotron Mössbauer spectroscopy at Sector 3 ID-B of the Advanced Photon Source at Argonne National Laboratory. We compressed orthoenstatite-structured (Mg,57Fe)SiO3 in diamond anvil cells to over one megabar and then used a CO2 laser to synthesize the material off-line into a post-perovskite structure. Evaluation of the synchrotron Mössbauer data provides the electric field gradient (quadrupole splitting) and s-electron density (isomer shift) of the iron sites, which in turn provides constraints on the valence and spin state of iron in these materials. We will discuss the implications of our results to Earth's D" layer.
MR22A-06
Dynamical effect of the spin transition: A numerical simulation study
Recent progresses in mineral physics have revealed several new phenomena under high-P,T conditions which likely affect chemical and physical properties of the Earth's lower mantle. In particular, spin transition of iron in the mantle minerals is one of the quaintest and also most important. Theoretical free energy modelling conbining with density functional computation proposed broad pressure region where high-spin and low-spin iron coexist along the mantle geotherm. Across this high-spin/low-spin mixed state, any properties are expected to vary continuously. This transient area is also confirmed experimentally. However, the dynamical effect of the spin crossover is still little understood. In this study, we have performed thousands atom numerical simulations to model the mixed spin state. Calculations suggest important consequences that the spin transition is accompanied by anomalous behaviors driven by dynamical fluctuation of spin state even though the structure remains macroscopically isosymmetric. Research supported by the Ehime Univ Project Fund and in part by JSPS.
MR22A-07
On the body centered structure of iron at Earth Core Conditions
The stability of the body-centered cubic (bcc) phase of iron is considered at Earth-core conditions (V = 7.2 Å3, T = 5500 K) by way of ab-initio molecular-dynamics simulations and electronic-structure calculations using the density-functional approach. Tetragonal strain splits a peak in the electronic density of states at the Fermi level, lowering the energy of the structure with distortion and therefore documenting the metastability of the bcc structure. As temperature is increased from zero to 5500 K, the bcc structure satisfies the condition of hydrostaticity, with vanishing of stress anisotropy, but it remains energetically unstable with respect to the tetragonal distortion; an increase in elastic anisotropy is documented with increasing tetragonal strain. An energy minimum is observed around c/a = 0.9 in a Bain-path plot, and has previously been interpreted as evidence of an elastically-stable body-centered tetragonal (bct) structure. However, we find from static total-energy calculations as a function of b/a and c/a that this is a local minimum in energy corresponding to a metastable structure. Analysis of the structure with tetragonal strain reveals the presence of anomalous shear stresses that, coupled with phonon instability, suggest a tendency toward formation of a hexagonal closed-packed (hcp) structure. Overall, these results argue against bcc or even bct structures of Fe being present in Earth's core, and instead highlight the importance of the known hcp (ε) high-pressure phase of iron.
MR22A-08
Effect of alloying elements on the magnetic transitions in iron-rich alloys
Pressure induced spin-pairing transitions occur in iron-bearing alloys, oxides, and silicates and affect the physical properties and chemical behavior of the Earth???¨º?¨¨s deep interior. Compared with diluted systems such as lower mantle magnesian silicate perovskite and ferro-periclase, loss of magnetic moments in iron-rich core alloys occurs at much lower pressure, and may involve different mechanisms from high- to low-spin transition in iron. We have measured the compression curve of a natural cohenite sample up to 56 GPa, using diamond anvil cells and synchrotron x-ray diffraction techniques at HPCAT, Argonne National Laboratory. In this study, we examine existing x-ray emission spectroscopy, synchrotron M?ssbauer spectroscopy, and x-ray magnetic circular dichroism data in order to understand the effect of H, C, P, S, Si, and Ni on the nature of magnetic transitions in iron-rich alloys. Combined with new and existing nuclear resonant inelastic x-ray scattering and x-ray diffraction data, we investigate the systematic effect of magnetic transitions on the density and velocities of these alloys.
MR22A-09
Phase relations of iron determined by in-situ x-ray diffraction in an internally-heated diamond anvil cell
Iron, as the primary phase of the Earth's core has been extensively investigated for the last 50 years. However, iron phase diagram is still controversial above 20 GPa which is beyond the multianvil apparatus pressure range. The major controversial issues include the Clapeyron slope of the FCC-HCP boundary, the presence of beta phase, and the melting curve, because of the lack of in-situ pressure determination at high temperatures in a diamond anvil cell (DAC) and technical challenge to generate stable and uniform heating in the DAC. In this study, we investigated the iron phase diagram using an improved ginternal-heatingh technique combined with in- situ X-ray diffraction measurements. High-pressure was generated with a pair of diamond anvils with 350- μm culet, using a gasket made of a fine powder of cubic boron nitride. In the sample chamber, ground quartz was used as a pressure medium and corundum was put on the iron foil. A thin (~15-μm) iron (99.999%Fe) foil was used as a sample and a heater simultaneously. The iron foil is heated up by increasing power through a DC power supply. This technique provides more stable and uniform heating than the laser- heating technique and much higher temperature than the external-heating method. High-P-T in-situ x-ray diffraction experiments were performed at the beamline 13-IDD of Advanced Photon Source. Temperatures were measured by the spectroscopic method and pressures at high temperature were determined from existing equations of state of Fe, SiO2, and Al2O3. We observed FCC-HCP transition in the pressure range from 20 to 50 GPa and constructed the phase relations of iron up to 50 GPa and 2000 K based on the data obtained by our new heating technique.